*3.3. Performance Testing of Solid Ceramic End Mills*

The operational tests' main task was to compare the efficiency of two options of end mills: (1) experimental specimens of SiAlON (80% (90α10β) + 20% TiN) ceramics obtained by spark plasma sintering and (2) industrial designs of commercial ceramic mills based on SiAlON, alloyed with stabilizing additives containing Yb. Another important task was to assess the prospects for using DLC coatings to increase these tools' wear resistance when processing heat-resistant nickel alloys. For the tests, groups of ceramic end mills of identical design were used (Figure 2). Section 2.7 details the performance test procedure, cutting data, and material properties. Figure 16 shows the dependences of flank wear on the operating time of two variants of ceramic end mills before and after DLC coating deposition. It can be seen that the experimental end mills developed within the framework of this work (1) are quite workable, but there is relatively intense wear of their cutting teeth, and they are noticeably inferior in operating time until critical wear (resistance) is reached compared to the reference specimens (2) commercial ceramic mills (5 min versus 7.1 min, respectively). Simultaneously, the spread of wear values for a ceramic end mill's teeth for commercial ceramic mills during operation is noticeably smaller. Such differences in the two types of ceramic end mills' tool life are not unexpected if we consider the experimental data presented in Figure 10. The comparative analysis of the microstructures of two variants of ceramics performed earlier showed that for the experimental samples sintered by the authors, there is a less uniform distribution of components over the volume of the sintered ceramic and a larger average grain size compared with commercial SiAlON ceramic doped with stabilizing additives containing Yb. It is well known that these characteristics are critical for tool ceramics and largely determine the tool's performance, especially under conditions of exposure to cyclic loads typical for milling. The DLC coating deposition makes it possible to reduce the wear rate of the experimental ceramic end mills (3), produced by the authors of the work, and demonstrates an increase in durability by 1.5 times, which is 7.7 min, relative to the uncoated cutting tool; at the same time, the spread of wear values on the ceramic end mill teeth slightly decreased. An important observation that can be made is that after DLC coating, the experimental end mills' durability is comparable to commercial ceramic mills' service lives. It cannot be considered an irregular or random result since the DLC coating deposition on commercial ceramic mills (4) demonstrates the same tendency: the coatings reduce the tool's wear rate and increase its tool life by 1.5, which is 11.2 min. Separately, it should be noted that the phenomena of pronounced chipping of cutting teeth of ceramic end mills were not observed in any of the tool's tested samples when carrying out resistance tests. The increase in the resistance of experimental samples of ceramic mills and commercial ceramic mills after DLC coating deposition established in this work is undoubtedly a consequence of the changes in the properties of the surface and surface layer of ceramics described in detail in Section 3.2.

Additionally, the work evaluated DLC coatings' effects on the surface quality (surface roughness) of machined nickel alloy NiCr20TiAl workpieces (Figure 17). The analysis of the presented curves, which were obtained based on the test results of ceramic end mills without coatings, shows that the height of the formed surface microroughness on the workpiece's surface with the tool operation course has a non-monotonic character

and characteristic peaks. Simultaneously, the curves obtained for experimental samples of ceramic end mills (1) and commercial ceramic mills (2) have a similar character. Considering that the level of antifriction properties of the tool surface strongly affects the quality of the workpiece surface layer, the nature of the experimental dependences of the processed workpiece surface roughness on the operating time is explained and in good agreement with the research results presented in Figure 15 (stepwise change with time in the friction coefficient of uncoated SiAlON-based ceramics under heating conditions up to 800 ◦C). Another regularity in the change in surface roughness can be observed during the operation of experimental (3) and commercial ceramic mills (4) after DLC coating deposition. The coating makes a pronounced contribution to improving the surface layer's quality of the workpiece being processed by changing the conditions of interaction with the cutting tool in the tribocontact zone. In the first minutes of operation, the surface roughness even slightly decreases and then begins to grow steadily. It is explained by the increase in the cutting tool wear zone and the actual area of its contact with the workpiece over time. Under such conditions, the intensity of their frictional and adhesive interaction increases, and the DLC coating contribution is leveled over time. Note that the described regularity correlates with the results of time variation of the friction coefficient SiAlON ceramic with a DLC coating under heating conditions up to 800 ◦C.

**Figure 16.** Dependence of flank face wear on the operating time of ceramic end mills before and after DLC coating deposition: (1) 80% (90α10β) + 20% TiN; (2) commercial ceramic mills; (3) 80% (90α10β) + 20% TiN with DLC coating; and (4) commercial ceramic mills with DLC coating.

**Figure 17.** Dependence of the processed surface roughness on the operating time of ceramic end mills before and after DLC coating deposition: (1) 80% (90α10β) + 20% TiN; (2) commercial ceramic mills; (3) 80% (90α10β) + 20% TiN with DLC coating; and (4) commercial ceramic mills with DLC coating.

### **4. Conclusions**

The complex of experimental studies carried out allows obtaining several original results that can be used to develop further the science-intensive direction associated with the creation and maintenance of effective operating conditions for solid end mills made of ceramics based on SiAlON powder compositions for machining heat-resistant nickel alloys.


on commercial ceramic mills demonstrates a similar result when the tool's wear rate decreases and its tool service life increases by 1.5 times and is 11.2 min. It is important to note that the coating contributes to improving the quality of the surface layer of the workpiece being processed by changing the conditions of interaction with the cutting tool in the tribocontact zone. In the first minutes of operation, the workpiece's surface roughness even slightly decreases and then begins to grow steadily. It is explained by the increase over time in the cutting tool wear area (land) and the actual area of its contact with the workpiece being processed. At the moment of failure, it is about 0.7 μm.

5. Deposition of (CrAlSi)N/DLC coating can be considered a promising way to improve the performance of ceramic end mills. Nevertheless, it is impossible to cover all issues and study the behavior of solid end mills made of ceramics in a wide range of changes in operating conditions within the framework of one work. First of all, research is needed at increased cutting speeds, which should be the next step in developing this scientific direction since it is possible to test the efficiency of (CrAlSi)N/DLC coating under high-speed milling conditions of heat-resistant nickel alloys only experimentally. Besides, it is necessary to search and test the effectiveness of new options for coatings and other surface finishing methods of sintered ceramics, which could significantly improve the properties of the surface and surface layer of ceramic end mills and thereby positively affect the performance of the tool.

**Author Contributions:** Conceptualization, S.N.G.; methodology, M.A.V.; validation, S.V.F. and A.A.O.; formal analysis, P.M.P., P.Y.P. and A.E.; investigation, P.M.P., P.Y.P. and A.E.; resources, P.M.P. and P.Y.P.; data curation, A.A.O. and A.E.; writing—original draft preparation, M.A.V.; writing review and editing, S.N.G.; visualization, A.A.O.; supervision, M.A.V.; project administration, S.N.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was supported by a grant of the Russian Science Foundation (project No. 21-79-30058).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are openly available in Figure 1a at (https://www.mmsonline.com/blog/post/applications-advance-for-solid-ceramic-end-mills, (accessed on 29 April 2021); https://www.cnctimes.com/editorial/solid-ceramic-endmills-fromkennametal-help-meet-critical-delivery-date, (accessed on 29 April 2021)), reference number [7,8]).

**Acknowledgments:** The work was carried out in the High-efficiency processing department of Moscow State University of Technology "STANKIN".

**Conflicts of Interest:** The authors declare no conflict of interest.
